1. Field of the Invention
This invention generally relates to a radio-conductive material which is a photoconductive material sensitive to radiations, a method of manufacturing such a radio-conductive material, a solid sensor which is provided with radio-conductive material layer on which image information can be recorded as a pattern of electrostatic charges (an electrostatic latent image) formed upon exposure to radiations, a method of manufacturing a film of such a radio-conductive material, and a radiation image read-out apparatus using such a solid sensor.
2. Description of the Related Art
In order to reduce irradiation dose to the patients and/or to improve diagnostic performance of the radiation image in a medical radiography, there have been proposed various systems in which a photoconductive body sensitive to a radiation (e.g., a-Se (amorphous selenium) plate) is used as an electrostatic recording medium, and an electrostatic latent image formed on the photoconductive body upon exposure to a radiation is read out by a laser beam or a number of electrodes. For example, see U.S. Pat. Nos. 4,176,275, 5,268,569, 5,354,982, and 4,535,468, xe2x80x9c23027 Method and Device for recording and transducing an electromagnetic energy patternxe2x80x9d; Research Disclosure June 1983, Japanese Unexamined Patent Publication No. 9(1997)-5906, U.S. Pat. No. 4,961,209, and xe2x80x9cX-ray imaging using amorphous seleniumxe2x80x9d; Med Phys. 22(12).
These systems are higher in resolution than known fluoroscopy using a TV pickup tube and less in irradiation dose to the patients than xeroradiography.
The radio-conductive material for forming the photoconductive body of the systems described above should be high in resistance in the dark, withstand a high electric field (e.g., 105 to 106 Vcmxe2x88x921), be high in radiation absorbance and be able to establish a high electric charge. Further the radio-conductive material should be able to form a film so that electric charges can move in the film without being trapped.
However the selenium radio-conductive materials which have been generally used in the conventional systems are difficult to form a film thin enough to prevent electric charge trapping and are not satisfactory in radiation absorbance though they are excellent is durability and able to establish high electric charges. Further since selenium is designated as poison by law, it is preferred that the radio-conductive material does not contain selenium.
As substitute radio-conductive material for selenium, inorganic/organic composite material formed of organic material and VB-VIB, VB-VIIB, IIB-VIB, IIB-VB, IIIB-VB, IIIB-VIB, I B-VIB or VB-VIIB inorganic material is disclosed in U.S. Pat. No. 5,556,716. For example, composite radio-conductive materials formed of inorganic material selected from the group consisting of BiI3, PbI4, PbI3 and Bi2S3 and organic material selected from the group consisting of nylon 11, PVK (N-polyvinyl carbazole) and PMMA (polymethyl methacrylate) are disclosed in the patent.
That BiI3/nylon 11 (50%/50% by weight) is a radio-conductive material which exhibits good radio-conductive properties is described in xe2x80x9cScience, 273(1966), 632xe2x80x9d.
The prior arts described above make it feasible to employ heavy element compounds, such as BiI3 which have been said to be not suitable as radio-conductive material since they are difficult to form a high quality film in a large area, are large in dark current and accordingly are not able to withstand a high electric field though being excellent in radiation absorbance, as radio-conductive material by forming a composite of such heavy element compounds with organic material (high-molecular material) which is relatively easy to form a high quality thin film, is small in dark current and is excellent in dielectric properties.
However the inorganic/organic composite materials are disadvantageous in that dispersion of the inorganic material (inorganic fine particles) in the organic material is apt to be deteriorated. That is, since the inorganic/organic radio-conductive materials are generally manufactured by so-called xe2x80x9cmelt deposition processxe2x80x9d in which organic material 81 is melted on a substrate 83 (FIG. 8) heated by a hot plate 82, inorganic particles are added to the molten organic material 81, and the mixture is stirred by, for instance, a spatula to form a film, it is difficult to uniformly disperse the inorganic particles in the organic material. When dispersion of the inorganic particles is not satisfactory, agglomerates of the inorganic particles are left in the formed thin film. The agglomerates deteriorate radio-conductive properties and durability to high electric fields, and can cause electric charge trapping.
Further, since, in the inorganic/organic composite materials, the radio-conductive inorganic material is xe2x80x9cdilutedxe2x80x9d by the organic material, the inorganic material content should be high in order to ensure a satisfactory radiation absorbance. For example, in the case of BiI3/nylon 11, the BiI3 content should be not smaller than 65 wt % in order to obtain a radiation absorbance equivalent to the conventional radio-conductive material of selenium under normal diagnostic radiographic conditions. In known combinations of inorganic material and organic material, it is impossible to uniformly disperse the inorganic material in such a high content, and accordingly, the radio-conductive properties of the radio-conductive material obtained cannot be high.
Further, when a thin film such as a radio-conductive layer is formed of the inorganic/organic radio-conductive material, voids can be embedded in the film, which reduces the inorganic material content in the inorganic/organic radio-conductive material and further deteriorates the radio-conductive properties of the radio-conductive material. Further the voids can deteriorate transfer efficiency of electric charges formed upon exposure to radiations.
Further, since the inorganic/organic composite radio-conductive material is in the form of a nano-composite in which the inorganic material is in the form of fine particles of several nanometers to several tens of nanometers, fine unevenness exists on the surface of a solid sensor formed of the inorganic/organic composite radio-conductive material, and accordingly, an electrode formed on the surface of the solid sensor, for instance, by deposition of Au cannot microscopically be in close contact with the radio-conductive layer. That is, gaps are microscopically formed between the electrode. Such gaps form charge wells in which electric charges formed in the radio-conductive layer concentrates, which can cause a stray current.
In view of the foregoing observations and description, the primary object of the present invention is to provide a radio-conductive material in which inorganic material is uniformly dispersed in organic material and which is good in radio-conductive properties and withstands a high electric field.
Another object of the present invention is to provide a method of manufacturing such a radio-conductive material.
Still another object of the present invention is to provide a solid sensor for recording a latent radiation image provided with a radio-conductive layer formed of a radio-conductive material which is good in radio-conductive properties, withstands a high electric field and is free from electric charge trapping.
Still another object of the present invention is to provide a method of manufacturing a radio-conductive film which is formed of inorganic/organic composite radio-conductive material and which is extremely small in void volume and accordingly large in inorganic material content.
Still another object of the present invention is to provide a solid sensor which is provided with a radio-conductive layer formed of inorganic/organic composite radio-conductive material and is free from a charge well which can cause a stray current.
That is, in accordance with a first aspect of the present invention, there is provided a radio-conductive material comprising alcohol-soluble nylon and inorganic material having radiation absorbing power.
In this specification, xe2x80x9cradiationxe2x80x9d means, for instance, X-rays, xcex3-rays and the like.
It is preferred that the inorganic material be bismuth iodide.
The alcohol-soluble nylon is a nylon which is solid at normal temperatures and is soluble in alcoholic solvent. For example, the alcohol-soluble nylon may be copolymer nylon obtained by copolymerizing various dibasic acids and diamines or nylon obtained by introducing an N-alkoxymethyl group into a polyamide binding of nylon. It is preferred that the alcohol-soluble nylon be composite material of nylon 6 and nylon 66.
It is further preferred that the radio-conductive material of the first aspect of the present invention be in the form of nano-composite.
In accordance with a second aspect of the present invention, there is provided a method of manufacturing radio-conductive material comprising the steps of dissolving alcohol-soluble nylon and inorganic material having radiation absorbing power in alcohol, and evaporating the alcohol to obtain high-viscosity composite material. This method will be referred to as xe2x80x9cliquid deposition processxe2x80x9d, hereinbelow. The high-viscosity composite material thus obtained is formed into a film or the like.
In accordance with a third aspect of the present invention, there is provided a solid sensor characterized by having a radio-conductive layer formed of the radio-conductive material in accordance with the first aspect of the present invention.
In the radio-conductive material in accordance with the first aspect of the present invention, since alcohol-soluble nylon is employed as the organic material and the organic material can be dissolved in alcohol, which facilitates to uniformly dispersing the inorganic material. Accordingly, the radio-conductive material can be a radio-conductive material which has both advantages of inorganic material, i.e., it is excellent in radiation absorbance, and advantages of organic material, i.e., it can be formed into a high quality film. Since being free from agglomerates of the inorganic particles, a film (e.g., a radio-conductive layer) formed of the radio-conductive material of the first aspect of the present invention can be better in radio-conductive properties, is less apt to cause electric charge trapping, and can be small in dark current which flows while the film is not being exposed to radiations.
Further, since the inorganic material can be uniformly dispersed in the organic material even if the organic material content is high, the radio-conductive properties of the film can be more improved.
Further, in accordance with the method of manufacturing the radio-conductive material of the third aspect, handling properties are improved and stability of the radio-conductive material manufactured can be highly improved.
In accordance with a fourth aspect of the present invention, there is provided a radio-conductive material represented by the following formula (I),
xe2x80x83BiI3/x.MX/y.nylonxe2x80x83xe2x80x83(I),
wherein M represents at least one alkali metal selected from the group consisting of Li, Na, K, Rb and Cs, X represents at least one halogen selected from the group consisting of F, Cl, Br and I, and x and y respectively represent the ratios by weight of MX and nylon to BiI3, x being 0 less than xxe2x89xa61, and y being 0 less than yxe2x89xa64.
It is preferred that MX be potassium fluoride.
It is further preferred that the radio-conductive material of the fourth aspect of present invention be in the form of nano-composite.
It is preferred that the nylon be alcohol-soluble. The alcohol-soluble nylon is a nylon which is solid at normal temperatures and is soluble in alcoholic solvent. For example, the alcohol-soluble nylon may be copolymer nylon obtained by copolymerizing various dibasic acids and diamines or nylon obtained by introducing an N-alkoxymethyl group into a polyamide binding of nylon. It is preferred that the alcohol-soluble nylon be composite material of nylon 6 and nylon 66.
It is preferred that the alkali halide represented by MX be alcohol-soluble.
Preferably 0 less than xxe2x89xa60.2 and 0.1 less than yxe2x89xa61.
In accordance with a fifth aspect of the present invention, there is provided a solid sensor characterized by having a radio-conductive layer formed of the radio-conductive material in accordance with the fourth aspect of the present invention.
In the radio-conductive material in accordance with the fourth aspect of the present invention, the alkali halide (MX) adheres to the surface of the bismuth iodide and changes the surface profile of the bismuth iodide, whereby the ratio of the bismuth iodide content to the nylon content can be increased. This means that the radiation absorbance of the radio-conductive material can be increased since the component of the radio-conductive material having radiation absorbing power is bismuth iodide.
Further, since the alkali halide changes the surface profile of the bismuth iodide, the radio-conductive material can be easily formed into a film even if the bismuth iodide content is high. Further, since dispersion of bismuth iodide is good, the radio-conductive layer of a solid sensor formed of the radio-conductive material of the fourth aspect of the present invention satisfactorily withstands a high electric field and is less in charge trapping.
Though the thin film of radio-conductive material is generally saturated with charges as the voltage applied to the film is increased, the film of the radio-conductive material in accordance with the fourth aspect of the present invention is hard to saturate with charges. It may be conceived that this is because there is no place where electric charges are gathered such as agglomerates of the inorganic particles in the formed thin film, and because the surface profile of the radio-conductive layer is changed. When the film is not saturated with electric charges with increase in the applied voltage, the radio-conductive properties of the solid sensor can be improved.
In accordance with a sixth aspect of the present invention, there is provided a method of manufacturing a radio-conductive film of an inorganic/organic composite radio-conductive material comprising the step of pressing the inorganic/organic composite radio-conductive material.
The radio-conductive material before pressing may be in the form of a block or in the form a film.
It is preferred that the inorganic/organic composite radio-conductive material be pressed at an elevated temperature. Preferably the elevated temperature be in the range of 50xc2x0 C. to 200xc2x0 C., and more preferably 120xc2x0 C. to 190xc2x0 C.
It is preferred that the inorganic/organic composite radio-conductive material be pressed at not higher than 50 Kg/cm2.
In accordance with a seventh aspect of the present invention, there is provided a method of manufacturing a radio-conductive film of an inorganic/organic composite radio-conductive material comprising the step of heating a film of inorganic/organic composite radio-conductive material.
It is preferred that the inorganic/organic composite radio-conductive material be BiI3/nylon.
In accordance with the methods of manufacturing a radio-conductive film of the sixth and seventh aspects of the present invention, the radio-conductive film formed can be extremely small in void volume and accordingly can be better in radio-conductivity.
In accordance with an eighth aspect of the present invention, there is provided a solid sensor comprising a radio-conductive layer formed of inorganic/organic composite material and an electrode provided on the radio-conductive layer, wherein the improvement comprises that the electrode is of indium.
In accordance with a ninth aspect of the present invention, there is provided a radiation image read-out apparatus comprising a solid sensor in accordance with the eighth aspect of the present invention and a read-out means for reading out a radiation image recorded on the solid sensor as a latent radiation image.
Indium is the softest in metals which are solid and stable at normal temperatures, is superior in processing characteristics, and can be almost freely deformed. Accordingly, the indium electrode can be in close contact with the radio-conductive layer formed of inorganic/organic composite material without any gap formed therebetween, whereby no charge well is formed in the solid sensor and the radio-conductivity of the solid sensor can be improved.